1. Field of the Invention
The present invention relates to an automatic gain control circuit for automatically controlling the gain of an amplifier in accordance with an input signal and supplying a stable amplifier output.
2. Description of the Prior Art
A conventional automatic gain control circuit performs coarse gain control by analog gain control, and fine gain control by digital gain control.
The hybrid automatic gain control circuit disclosed in Japanese Unexamined Patent Publication No. 55-97714 will be described below with reference to FIG. 1. A gain selector 401 coarsely controls the gain of a register input signal S.sub.in in accordance with an output signal S.sub.RO1 from a register 402 to output an analog gain control output signal S.sub.agco. The register 402 stores a gain control signal S.sub.C1 and outputs the register output signal S.sub.RO1. An A/D converter 404 converts the analog gain control output signal S.sub.agco into a digital signal and outputs it as an A/D output signal S.sub.ado. A register 406 stores a gain control signal S.sub.C2 and finely controls the gain of a multiplier 405 in accordance with a register output signal S.sub.RO2 to output a digital gain control output signal S.sub.dgco. A gain control circuit 403 receives the digital gain control output signal S.sub.dgco from the multiplier 405, and outputs the gain control signal S.sub.C1 for coarse gain control and the gain control signal S.sub.C2 for fine gain control. In this manner, automatic gain control is performed to keep the input signal to the demodulation circuit constant.
Owing to the above circuit arrangement of the conventional automatic gain control circuit, as the loop gain increases, an error increases, whereas as the loop gain decreases, the control processing speed decreases. To realize high-speed and stable control, the loop gain must be controlled. In addition, the circuit constituted by the analog gain control section for coarse gain control and the digital gain control section for fine gain control becomes complicated.